Hi Giorgio, I also suspect the presence of twining and as Eleanor has suggested the L test is the best test to detect that (if also t-NCS is not present). You can reprocess your data in P1, do MR in P1 and then feed your P1 model and mtz file to zanuda to see what SG it suggests. That trick once worked for me.
Cheers, Sudipta. Sudipta Bhattacharyya, Postdoctoral fellow Department of Molecular Biosciences University of Texas at Austin Texas, USA. On Tue, Aug 8, 2017 at 8:27 AM, Eleanor Dodson <eleanor.dod...@york.ac.uk> wrote: > First of all - check whether your crystal data shows twinning - thee is an > L-test plot which usually gives a clear indication and if you are using > GUI2 the report suggests if this is so. > > > If you have twinning the most likely SG is P31 (or P32 ) - you cant tell > the difference till the structure is solved. > > Check the merging stats in that point group - > Then try MR searches in the two spacegroups. > > The C2 and I2 SGs are probably sub-group possibilities if you reindex - > there are common cell dimensions -pointless output tells you how to do that > - but in general one always chooses the highest symmetry that gives > reasonable merging statistics and is sensible. > > Eleanor > > > > On 8 August 2017 at 14:15, Giorgio Giardina <giorgio.giard...@uniroma1.it> > wrote: > >> Hello everybody, >> >> I think I have a pseudo-centering problem. >> >> I have a 1.6 ang. dataset of a mutant protein that is an homodimer in >> solution. >> Data processing gives the following SG: >> >> Space group: P 31 2 1 >> Average unit cell: 111.36 111.36 28.56 90.00 90.00 120.00 >> Average mosaicity: 0.24 >> Rmerge Overall 0.07 >> >> However with this SG the unit Cell is too small and monomer doesn't fit >> the in the AU. >> >> I reprocessed the data in other possible SG (including P6) and finally I >> got 2 equivalent SG's in which I can get a correct Molecular Replacement >> solution: >> >> Space group: I 1 2 1 >> Average unit cell: 57.07 111.11 192.90 90.00 90.07 90.00 >> >> and >> >> Space group: C 1 2 1 >> Average unit cell: 201.10 111.11 57.07 90.00 106.42 90.00 >> >> Both with Average mosaicity: 0.38 and Rmerge Overall 0.06, >> but the corresponding MR solutions do not refine, with R-factor stuck to >> 45-47%. >> >> From what I understand, in the I121 SG I have the NC two-fold axis of the >> dimer at 1/2a and this originates the pseudo centering and the small P3 Cell >> >> Largest Patterson peak with length larger than 15 Angstrom: >> Frac. coord. : 0.500 0.000 0.000 >> Distance to origin : 28.566 >> Height relative to origin : 53.830 % >> >> I'm really not so good with symmetry, so I'll be grateful for any >> suggestion/help/solution from you out there. >> Many thanks, >> Giorgio >> > >
